Studies are underway to examine the mechanisms by which species of oral Actinomyces adhere to oral Streptococcus species and mammalian cells. The actinomycetes may contribute to the etiology of periodontal disease and it is significant that many human strains carry a lactose inhibitable receptor which seems to account for both coaggregation with certain streptococci and hemagglutination with human erythrocytes. This receptor is associated with surface fibrils (i.e. fimbriae) on the actinomycetes and efforts are being made to identify a fibrillar antigen(s) which contains the binding site. Coaggregation of various strains of A. viscosus and A. naeslundii with S. sanguis strains appears to depend on a system of specific cell surface interactions which can be used to group these bacteria. This system may contribute significantly to the distribution of these microorganisms within the oral cavity and is being investigated.